(Circulation. 1999;100:II-357.)
© 1999 American Heart Association, Inc.
Myocardial Protection and Vascular Biology |
Opiate Drugs and 
-Receptor–Mediated Myocardial Protection
From the University of Michigan Health System (P.E.B., S.F.B.) and St. Joseph Mercy Hospital (M.B.B.), Ann Arbor, Mich; and the National Institutes of Health, Bethesda, Md (T.-P.S.).
Correspondence to Patrick E. Benedict, MD, Department of Anesthesiology, University of Michigan Health System, 1500 E. Medical Center Drive, 1G323 Box 0048, University Hospital, Ann Arbor, MI 48109-0048. E-mail pbenedic{at}umich.edu
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Abstract |
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Background—Hypothermic myocardial arrest is necessary to complete most cardiac surgery, which limits the success of such operations. Similarly, cold, inhospitable environments limit the survival of warm-blooded animals. Animals have successfully adapted to this challenge through hibernation. Hibernation is an energy-conserving state, now known to be governed by cyclical variation in endogenous opiate compounds. It may also be induced in nonhibernators via hibernating animal serum factors or
-opiate peptides. Furthermore, hibernation-induction triggers
extend organ preservation in many models. This study examined whether
opiate drugs with an affinity for the -opiate receptor confer
similar protection.
Methods and Results—Isolated hearts harvested from New Zealand
White rabbits were treated with either cardioplegia alone or -opiate
drugs (fentanyl, morphine, buprenorphine, pentazocine) followed by 2
hours of 34°C ischemia. Hearts were then reperfused, and
functional and metabolic indices of treated groups were
compared with untreated controls. Isovolumic developed pressure,
coronary flow, and oxygen consumption were compared as a
percent of preischemia versus 45 minutes after reflow.
Developed pressure and oxygen consumption were better preserved in the
morphine, buprenorphine, and pentazocine groups when compared with
cardioplegia alone.
Conclusions—Drugs with -opiate activity confer myocardial
protection, which is additive to cardioplegia. Use of -opiate drugs
in this context may have important clinical implications.
Key Words: cardioplegia • cardiopulmonary bypass • drugs • hibernation • receptors
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Introduction |
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Despite years of experience, clinicians are still challenged by the perioperative care of the cardiac patient. Most operations on the heart require the use of hypothermic, ischemic cardiac arrest, which limits the success of such operations. Similarly, warm-blooded animals are challenged by life in cold, inhospitable environments. Many animals have successfully adapted to this challenge via hibernation. A fascinating concept is that the biological mechanism of hibernation may be duplicated in humans, thereby inducing a profound state of energy conservation throughout the body or at the organ level. Conceivable applications for "induced hibernation" include transplantation and cardiac surgery.
Hibernation only occurs in certain animals, such as black bears,
woodchucks, and ground squirrels, in response to climatic conditions.
During this process, the metabolic processes of the body
are dramatically slowed down. In fact, hibernating animals use only
10% of their normal, active energy expenditure.1
Hibernation is a process mediated by cyclical variation in
endogenous opiate compounds.2 3 4 5 It is also
evident that hibernation is not only opiate-mediated, but that the
-opiate receptor in particular is responsible.5 6
Additionally, serum from hibernating animals, when injected into
summer-active animals, induces hibernation behavior and physiology.
Conversely, hibernation can be reversed by opiate
antagonists.5 6
Given that hibernation is a state of energy conservation and is
reproducible with the administration of -opiates, potential
implications for organ preservation arise. In fact, using hibernation
triggers to extend organ viability has been done successfully in many
models,7 8 9 10 11 including myocardial
protection.12 13 Coincidentally, nonpeptide, opiate drugs
are the mainstay of cardiac anesthetics. These drugs are used in this
setting for their potent analgesic properties and
cardiovascular stability. To some degree, each opiate
drug in common use is thought to possess an affinity for the -opiate
receptor.14 15 16 17 18 The hypothesis of the current
investigation was that opiate drugs with activity at the -receptor
will confer protection against ischemic insult in a fashion
similar to endogenous hibernation-induction triggers and
the synthetic -peptides.
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Methods |
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The model used in this study was an isolated, beating rabbit heart. Briefly, New Zealand White rabbits (male or female; body weight, 2.0 to 2.5 kg) were anesthetized with sodium pentobarbital (45 mg/kg IV) and heparinized (700 U/kg IV). Each heart was excised, and the aorta was immediately cannulated in the Langendorff mode. The heart was then perfused with a physiological salt solution (pH 7.4) containing (in mmol/L): NaCl 118, KCl 4.0, NaHCO3 22.3, glucose 11.1, KH2PO4 0.66, MgCl2 1.23, and CaCl2 2.38. The solution was equilibrated with 95% O2/5% CO2 at 37°C and passed twice through filters with a pore size of 3.0 µm. Perfusion pressure was maintained at 90 mm Hg.
A fluid-filled latex balloon was inserted into the left ventricle
through a small incision in the left atrium. The balloon was connected
to a pressure transducer for continuous measurement of left
ventricular pressure, and its first derivative,
dP/dT. Vena cavae and azygous veins were ligated. A cannula was
placed in the pulmonary artery to collect measurements of
coronary flow (CF) and oxygen content. Analog signals from the
above measurements were digitized to an online computer (AST Premium
386, AST Research Inc). Left ventricular function
was characterized as developed pressure, which was defined as peak
systolic pressure minus end-diastolic pressure.
Myocardial oxygen consumption
(M
O2) was calculated using the
following formula:
MO2=CF(cc ·
min-1 ·
g-1)x(O2 partial
pressure [mm Hg] difference between perfusate and
effluent)xBunsen solubility coefficient of O2 at
37°C (22.7 µL of O2 ·
atm-1 · cc of
perfusate-1/760). The partial
pressure of O2 of the perfusate was
665 mm Hg. CF was measured by collecting the coronary
effluent in a graduated cylinder. Oxygen extraction was calculated as
MO2 divided by
O2 content of the perfusate.
Optimal left ventricular balloon volume was determined after instrumentation was complete to insure that pressure measurements after interventions were within the appropriate compliance range for each experiment. The volume was adjusted such that developed pressure ranged from 100 to 140 mm Hg. This volume was then maintained during baseline and reperfusion conditions. Baseline data were recorded after a 30-minute stabilization and equilibration period. All isolated hearts were maintained in a 37°C water-jacketed organ bath during the entire baseline period. At the conclusion of the baseline period, the physiological salt solution infusion was stopped, and 60 cc of 4°C cardioplegia solution was injected into the aorta over 1 minute to commence the 2-hour 34°C period of ischemia.
In the treatment groups, the test drug was infused over 5 minutes at the end of the baseline equilibration period at a concentration thought to be clinically relevant (based on current human dosing). These concentrations were as follows: morphine 1 µg/cc, fentanyl 100 ng/cc, buprenorphine 500 ng/cc, and pentazocine 5 µg/cc. They were also equipotent. A total of 20 cc of each concentration was added to the physiological salt perfusate and delivered over 5 minutes. Each heart was allowed to re-equilibrate for 5 minutes before standard cardioplegia-induced ischemia. These hearts were then treated in a similar fashion to controls. At the conclusion of each experiment, each heart was harvested for histological examination and grading, which was accomplished via a semiquantitative visual scale.
Statistical analysis was performed using the Stat-View program (Abacus Concepts Inc). Data were evaluated using ANOVA (Scheffe’s test). Differences between groups were considered significant at P<0.05. All animal treatments were performed in compliance with the Principles of Laboratory Animal Care, formulated by the National Society for Medical Research. All work was performed in laboratories in the section of Cardiac Surgery at the University of Michigan Medical Center, Ann Arbor.
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Results |
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In the preischemia interval, no significant differences existed between groups in any of the functional parameters recorded. The Table
summarizes the outcome of the
different treatment groups in the postischemia interval.
Functional recovery was compared with untreated controls. Isovolumic
developed pressure, CF, and
MO2 were compared as a percent
of preischemia versus 45 minutes after reflow. Data are
mean±SEM.
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The functional recovery of each treatment group is expressed as the
percent of developed pressure after reperfusion compared with the
preischemic value. Metabolic recovery was
assessed in a similar fashion using
MO2. Isolated hearts in this
study showed a marked decrement in both functional and
metabolic integrity after 2 hours of ischemia.
Control hearts developed only 38% of their original
contractility on reperfusion. Opiate drugs used for
pretreatment in this study were associated with varying degrees of
functional improvement after global myocardial ischemia.
Fentanyl, which is generally regarded as a potent µ-receptor agonist,
offered no significant improvement in functional recovery. Morphine
pretreatment was associated with preserved functional recovery, which
implies activity at the -receptor in addition to its known
µ-affinity. Buprenorphine and pentazocine were chosen as treatment
drugs because of their theoretically higher affinity for -opiate
receptors. Pretreatment with these compounds resulted in an impressive
degree of myocardial protection against ischemic insult. On
histological examination, the functionally improved
groups also showed preserved ultrastructural integrity (intact
mitochondrial architecture and glycogen storage granules and less free
intracellular calcium) when compared with controls.
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Discussion |
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Previous investigations demonstrated that animal hibernation is a process due, in large part, to circannual variation of opiate-like compounds in endogenous serum.19 20 Specifically, the yet-to-be-identified endogenous
-opiate agonist, heretofore known as HIT (hibernation-induction
trigger), may be the responsible serum factor.5 Synthetic
compounds that resemble HIT in their affinity for this receptor have
also demonstrated efficacy in eliciting hibernation behavior and
physiology. Improved energy conservation and organ preservation have
been attributed to pretreatment with both HIT and synthetic
analogs.7 8 9 10 Specifically, hearts exposed to global
ischemia seem to be relatively protected from
postischemic dysfunction compared with those not receiving
preischemic conditioning via -opiate
compounds.12 13
We think that the -opiate receptor elicits cardioprotective effects
and, more specifically, that the 2-receptor is
responsible for the protection observed in our studies. Previous work
performed in our laboratory demonstrated the efficacy of natural
hibernation induction triggers and the synthetic compound
D-Ala2-Leu5-enkephalin (DADLE) in mediating myocardial protection that
is superior to cardioplegia. The synthetic peptide D-pen2,5-enkephalin
(DPDPE), a 1-opiate, failed elicit such
protection.13 The objective of the present
investigation was to bring our previous work, using synthetic peptides
and animal serum, 1 step closer to the clinical realm by using opiates
in common clinical use. Although data exist indicating that some of
these commonly used drugs are active at the
-receptor,15 16 21 22 no data exist regarding
their respective affinities for -subtypes. We included fentanyl in
the study as a contrast to the -compounds because of its known
strong µ-receptor affinity.15 Finally, various
investigations showed that morphine has an affinity at both µ- and
-receptors.21 These previous studies indicate the
binding affinities of the drugs used in this study for opiate receptor
subtypes, and we chose the drugs on the basis of these data.
Teleologically, the concept of energy conservation, at the whole body and organ level, makes sense for the mammalian species using it. Known hibernators, such as squirrels, bears, hedgehogs and woodchucks, use this process to conserve much of their energy at a time when temperatures are subfreezing and food is scarce.23 Profound behavioral and metabolic changes are hallmarks of this process, which includes hypothermia, bradycardia, respiratory depression, hypophagia, and behavioral changes resembling anesthesia or deep sleep.4 These changes allow prolonged survival despite the presence of extreme cold and the absence of nutritional intake for months at a time.
Dawe and Spurrier24 were the first to demonstrate an endogenous "trigger" responsible for hibernation. They showed that plasma from the hibernating ground squirrel, when injected in either summer-active ground squirrels or woodchucks, produced a behavioral and physiological state resembling hibernation. The identity of the endogenous hibernation trigger has been elusive, however. Recent evidence implicates a plasma protein that is thermolabile, protease sensitive, and nuclease insensitive.2 HIT seems to be albumin-related, and its manifestations in hibernating species may be dependent on cyclical variations in plasma albumin concentrations.25
Oeltgen et al6 were the first investigators to
successfully simulate hibernation in a nonhibernating organ
model. They showed an increase in transplant graft survival time
when using HIT to cause energy conservation in ischemic dog
organs. When canine donor lungs were preconditioned with serum from
hibernating woodchucks, a remarkable improvement in graft survival time
was demonstrated. The synthetic compound DADLE was at least as
effective as HIT in the same organ preparation.6 In
separate studies, Bolling et al12 13 used both the natural
HIT and the synthetic opiate DADLE to create significant myocardial
protection in an isolated rabbit heart model. Further studies using
receptor antagonists will provide more compelling evidence
that drug- and serum-induced cardioprotection is, in fact, due to
opioid receptor activation. Currently, work is in progress in our
laboratory using the -subtype specific antagonists
naltriben and naltrindole and the nonspecific opiate
antagonists naloxone and naltrexone to provide such
evidence.
In light of these recent discoveries, it is interesting and perhaps
fortuitous that opiates are the primary component of cardiac
anesthetics. Their popularity in this context stems from their potent
analgesic effects combined with a high degree of
hemodynamic stability. Given the fact that -opiates
govern hibernation-related energy conservation and that most opiate
drugs possess some -activity, their usefulness may extend beyond the
provision of patient comfort and stability during cardiac operations.
The drugs used in this study were chosen because they are commonly used
in humans and are thus clinically relevant. Hence, this work reflects
an effort to bring the concept of opiate-mediated energy conservation
closer to clinical human application.
Although it is conceivable that opiates may someday comprise a portion
of standard cardioplegic regimens, their use is not without potential
pitfalls. Side effects of the opiate drugs are well known, and they
include respiratory depression, nausea, pruritus, and sedation. These
would need to be taken into account if the drugs were given with the
specific goal of myocardial protection. It seems, however, that the
concentration necessary to achieve protective effects is well below
that achieved using standard intravenous analgesic
regimens. Morphine is a part of many cardiac premedication regimens,
and its use may pose a problem when present in concert with
-opiate agents used specifically for myocardial protection. As
previous studies have shown, morphine may antagonize -mediated
effects because it is an opiate µ-agonist,6 thus
negating the beneficial effects of -agents. However, this study and
others26 indicate that morphine itself has activity at the
2-receptor, which implies beneficial effects
with respect to myocardial protection. Finally, the use of
agonist/antagonist drugs to achieve myocardial protection
could antagonize the analgesic effects of µ-active agents like
morphine and fentanyl. This would need to be taken into account before
using opiates for such purposes.
Although both HIT and synthetic -peptides can mimic hibernation
physiology in animal models, neither compound is presently
applicable for human administration. Therefore, we attempted to
simulate this hibernation-like energy conservation by using common
drugs that possess -opiate activity. The exact mechanism of
-mediated energy conservation has yet to be elucidated, but recent
investigation points to several hypothetical pathways. Evidence exists
that preservation of intracellular ATP, through inhibition of adenosine
triphosphatases, may improve ischemic
tolerance.27 These agents may also protect myocyte
membranes against oxygen-derived free radicals or improve the release
of calcium from the sarcoplasmic reticulum.28 29 30 31 Altered
DNA and protein synthesis are also associated with hibernation
induction,32 and this may play a part in the profound
metabolic inhibition associated with -opioid activity.
Opioids may also inhibit stimulatory G-proteins33 or
affect energy conservation through a potassium-ATP
mechanism.26 At present, research continues in an
effort to identify and characterize HIT and elucidate its exact
mechanism of action. Specifically, we are examining the effects of
glyburide, a known potassium-ATP antagonist, and pertussis
toxin, a G protein antagonist, and their interaction with
opiate-related phenomena.
The present study demonstrated that pretreatment with commonly used
medications that have -opioid activity improved
postischemic metabolism and function in an
isolated rabbit heart model. Administration of the compounds before
global ischemia, which is followed by improvement in
postischemic recovery, is consistent with opiate
receptor activation, as demonstrated in the aforementioned studies
using HIT and peptide opiates. The enhancement of myocardial
preservation afforded by these compounds is similar in magnitude to
that of HIT and the synthetic opiate DADLE. This surgically analogous
investigation was not intended to create an infarct model. Rather, our
goal was to mimic the global stunning attributable to the use of
cardioplegia and ischemic arrest that is necessary to perform
cardiac surgery in humans. This work indicates that the compounds
morphine, buprenorphine, and pentazocine have an affinity for the
-opiate receptor and, thus, may have clinical utility in
ischemic organ preservation.
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